hailstorms are commoner in summer and in warm countries; the heat is
greater and it thrusts the clouds further up from the earth. But the
fact is that hail does not occur at all at a great height: yet it
ought to do so, on their theory, just as we see that snow falls most
on high mountains. Again clouds have often been observed moving with a
great noise close to the earth, terrifying those who heard and saw
them as portents of some catastrophe. Sometimes, too, when such clouds
have been seen, without any noise, there follows a violent
hailstorm, and the stones are of incredible size, and angular in
shape. This shows that they have not been falling for long and that
they were frozen near to the earth, and not as that theory would
have it. Moreover, where the hailstones are large, the cause of
their freezing must be present in the highest degree: for hail is
ice as every one can see. Now those hailstones are large which are
angular in shape. And this shows that they froze close to the earth,
for those that fall far are worn away by the length of their fall
and become round and smaller in size.

  It clearly follows that the congelation does not take place
because the cloud is thrust up into the cold upper region.

  Now we see that warm and cold react upon one another by recoil.
Hence in warm weather the lower parts of the earth are cold and in a
frost they are warm. The same thing, we must suppose, happens in the
air, so that in the warmer seasons the cold is concentrated by the
surrounding heat and causes the cloud to go over into water
suddenly. (For this reason rain-drops are much larger on warm days
than in winter, and showers more violent. A shower is said to be
more violent in proportion as the water comes down in a body, and this
happens when the condensation takes place quickly,-though this is just
the opposite of what Anaxagoras says. He says that this happens when
the cloud has risen into the cold air; whereas we say that it
happens when the cloud has descended into the warm air, and that the
more the further the cloud has descended). But when the cold has
been concentrated within still more by the outer heat, it freezes
the water it has formed and there is hail. We get hail when the
process of freezing is quicker than the descent of the water. For if
the water falls in a certain time and the cold is sufficient to freeze
it in less, there is no difficulty about its having frozen in the air,
provided that the freezing takes place in a shorter time than its
fall. The nearer to the earth, and the more suddenly, this process
takes place, the more violent is the rain that results and the
larger the raindrops and the hailstones because of the shortness of
their fall. For the same reason large raindrops do not fall thickly.
Hail is rarer in summer than in spring and autumn, though commoner
than in winter, because the air is drier in summer, whereas in
spring it is still moist, and in autumn it is beginning to grow moist.
It is for the same reason that hailstorms sometimes occur in the
late summer as we have said.

  The fact that the water has previously been warmed contributes to
its freezing quickly: for so it cools sooner. Hence many people,
when they want to cool hot water quickly, begin by putting it in the
sun. So the inhabitants of Pontus when they encamp on the ice to
fish (they cut a hole in the ice and then fish) pour warm water
round their reeds that it may freeze the quicker, for they use the ice
like lead to fix the reeds. Now it is in hot countries and seasons
that the water which forms soon grows warm.

  It is for the same reason that rain falls in summer and not in
winter in Arabia and Ethiopia too, and that in torrents and repeatedly
on the same day. For the concentration or recoil due to the extreme
heat of the country cools the clouds quickly.

  So much for an account of the nature and causes of rain, dew,
snow, hoar-frost, and hail.

                                13

  Let us explain the nature of winds, and all windy vapours, also of
rivers and of the sea. But here, too, we must first discuss the
difficulties involved: for, as in other matters, so in this no
theory has been handed down to us that the most ordinary man could not
have thought of.

  Some say that what is called air, when it is in motion and flows, is
wind, and that this same air when it condenses again becomes cloud and
water, implying that the nature of wind and water is the same. So they
define wind as a motion of the air. Hence some, wishing to say a
clever thing, assert that all the winds are one wind, because the
air that moves is in fact all of it one and the same; they maintain
that the winds appear to differ owing to the region from which the air
may happen to flow on each occasion, but really do not differ at
all. This is just like thinking that all rivers are one and the same
river, and the ordinary unscientific view is better than a
scientific theory like this. If all rivers flow from one source, and
the same is true in the case of the winds, there might be some truth
in this theory; but if it is no more true in the one case than in
the other, this ingenious idea is plainly false. What requires
investigation is this: the nature of wind and how it originates, its
efficient cause and whence they derive their source; whether one ought
to think of the wind as issuing from a sort of vessel and flowing
until the vessel is empty, as if let out of a wineskin, or, as
painters represent the winds, as drawing their source from themselves.

  We find analogous views about the origin of rivers. It is thought
that the water is raised by the sun and descends in rain and gathers
below the earth and so flows from a great reservoir, all the rivers
from one, or each from a different one. No water at all is
generated, but the volume of the rivers consists of the water that